1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and particularly to the start time control of an internal combustion engine that includes a mechanism for directly injecting fuel into a cylinder.
2. Description of the Background Art
From the standpoint of increasing the engine performance, a variable valve timing (VVT) mechanism that allows variable timing of opening/closing an intake valve and an exhaust valve is employed. On the basis of this VVT mechanism, there is proposed the control of reducing the effective compression ratio by retarding the valve timing of the intake valve at the time of starting the engine (hereinafter, also referred to as “start time decompression control”) in a hybrid vehicle whose engine that includes an in-cylinder injector (in-cylinder injection valve) is operated intermittently (for example, Japanese Patent Laying-Open No. 2000-64874; referred to as Patent Document 1 hereinafter). The start time decompression control is advantageous in that the initial expansion torque at the engine is reduced to suppress engine vibration and reduce cranking resistance to ensure smooth starting capability.
The technique of suppressing increase in the combustion variation is also disclosed for the in-cylinder injection type internal combustion engine. In the implementation of the intake valve delayed closing control in which the closing timing of the intake valve is set at the retarded side than the piston intake bottom dead center (Atkinson cycle), the pressure of fuel injection from the fuel injection valve is set higher or the fuel injection time more advanced than when the Atkinson cycle is not executed, whereby increase in the combustion variation is suppressed (for example, Japanese Patent Laying-Open No. 2004-52551; hereinafter referred to as Patent Document 2).
Further, there is proposed an internal combustion engine including a main fuel injection valve (in-cylinder injector) injecting fuel directly into a cylinder (into a combustion chamber) and an auxiliary fuel injection valve (intake manifold injector) injecting fuel into an intake manifold (for example, Japanese Patent Laying-Open No. 2001-73854; referred to as Patent Document 3 hereinafter). In this internal combustion engine, fuel injection from the main fuel injection valve (in-cylinder injector) is inhibited for a predetermined period of time from startup that is determined according to the coolant temperature of the engine. Accordingly, emission of unburned components at the time of starting the engine is reduced to suppress wasteful fuel consumption.
Execution of decompression control by the VVT mechanism at the time of starting an internal combustion engine that includes an in-cylinder injector as in Patent Documents 1 and 2 is disadvantageous in that the pressure and temperature at the compression end in the cylinder will be reduced although vibration is suppressed to allow the engine to be started smoothly. Vaporization of the fuel injected from the in-cylinder injector will become insufficient to pose the problem of increased exhaust emission due to deterioration in burning.
Although Patent Document 3 is related to measures against exhaust emissions at the time of starting an engine that includes both an in-cylinder injector and an intake manifold injector, it is completely silent about measures for improving the exhaust emission level during start time decompression control based on the VVT mechanism.